The threats of sabotage or acts of terrorism make plants, storage yards, etc. vulnerable. Security forces long have relied upon patrols or some sort of a beam interrupter or doppler apparatus for sensing an intrusion into an area. While these provide a degree of security, a family of more sophisticated and reliable approaches has been developed. All rely on the deployment of an elongate, hose-like structure which has a number of vibration sensitive devices located along its length. A noteworthy example of a system of this type is disclosed in U.S. Pat. No. 3,806,908 issued to Lloyd R. Bound on Apr. 23, 1974. The system uses a number of piezoelectric transducers and wires contained in or connected to a buried cable. Another pair of interesting approaches is disclosed in the U.S. Pat. No. 3,832,704 issued to Vahram S. Kardashian on Aug. 27, 1974 and the U.S. Pat. No. 3,882,441 issued to James O. Holmen et al on May 6, 1975. Both rely on the interaction of magnetostrictive film plated wires to sense external disturbances and the location of a possible intruder. Still another intrusion detection system was designed by Robert F. Bell et al and patented on Sept. 3, 1974 in U.S. Pat. No. 3,833,897. A coaxial electric cable converts mechanical vibrations to representative signals capable of being monitored at a remote station. The signals generated are distinctive sounds made by an intruder either climbing or cutting through a chain link fence. Provision is made for a loudspeaker or an alarm generator to indicate where an intrusion has occurred.
All of the aforeidentified detection systems represent meritorious advances in the state of the art; however, questions arise as to their reliability. Since all rely on metallic conductors powering the system and for returning information to a monitor, the problems associated with signal-to-noise ratios and crosstalk must be considered. Using coaxial cables is not the complete solution, since their bulk, expense and susceptibility to damage precludes their use in a vast number of situations.
Cables have been designed which combine the transmission of optical and conventional electrical power. An example of such a conduit is described in U.S. Pat. No. 3,660,590 issued to James E. Conant on May 2, 1972. The patent discloses fiber-optic as well as metallic conductors wrapped together to enclose a passageway for liquids. While this conduit is unique for its envisioned use and eliminates cross-talk problems while giving a hydraulic communication capability, it cannot detect an intrusion.
Thus, there is a continuing need in the state-of-the-art for an intrusion sensor line which advantageously employs the concept of combining sensors, fiber-optics and electrical power conductors for reducing the problems associated with signal-to-noise degradation and cross-talk while presenting a compact, easily deployable unit.